This invention relates to a cylindrical permanent magnet unit for producing a magnetic field in the direction of the longitudinal axis of the cylindrical unit within the bore of the cylindrical unit. Essentially the cylindrical magnet unit is an assembly of a plurality of ring-like permanent magnets magnetized in radial directions. The cylindrical magnet unit is suitable for application to gyrotrons and some other electron tubes such as gyro-travelling-wave tubes.
A gyrotron is an electron tube to generate a microwave by utilizing maser effects of cyclotron resonance. In a gyrotron a tubular beam of electrons interacts with an electromagnetic field in a resonant cavity, and the interaction results in conversion of the kinetic energy of electrons into electromagnetic energy and generation of a high-frequency wave. Known gyrotrons include gyromonotrons having a single resonant cavity and gyroklystrons having a plurality of resonant cavities to accomplish amplification of high-frequency waves.
For instance, JP-A 56-102045 shows a gyrotron apparatus with a single resonant cavity. The gyrotron has an electron gun at one end of a tubular body of the apparatus, and a middle section of the tubular body provides a resonant cavity. Outside of the tubular body, a cylindrical electromagnet surrounds the electron gun, and another cylindrical electromagnet surrounds the resonant cavity. In the bore of the tubular body a magnetic field in the direction of the longitudinal center axis of the bore is produced by the two electromagnets. The electrons drifting from the electron gun are affected by the magnetic field and make spiral motion, while the electrons form a tubular beam. The magnetic flux density in the gyrotron body gradually increases from the end section where the electron gun is positioned toward the resonant cavity. In the resonant cavity the distribution of flux density should be flat in the direction of the center axis. The arrangement of the two electromagnets and the magnet excitation currents are determined so as to realize the desired distribution of flux density.
The two electromagnets in the gyrotron apparatus are normal conductivity magnets or superconducting magnets, or a combination of a normal conductivity magnet and a superconducting magnet. In the resonant cavity a very strong magnetic field is needed for oscillation at a very high frequency. Usually normal conductivity magnets are used for oscillation at frequencies below about 30 GHz and superconducting magnets for oscillation at higher frequencies.
Superconducting magnets are generally very costly, and for excitation the magnets must be cooled to a very low temperature by using either a refrigerant such as liquid helium or a high-performance refrigirating apparatus. Besides, it is difficult to quickly vary the strength of a magnetic field produced by a superconducting magnet. Normal conductivity magnets for producing a very strong magnetic field need power supplies of very large capacity for excitation and consume very large power. Besides, it is necessary to cool the electromagnets and power supplies.